This application is based on Japanese Patent Application HEI 11-232546, filed on Aug. 19, 1999, the entire contents of which are incorporated herein by reference.
Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, particularly a semiconductor device with a polysilicon film.
A method of manufacturing a thin film transistor (TFT) using a polysilicon film will be described. An amorphous silicon film is formed on a glass substrate by chemical vapor deposition. An excimer laser beam is radiated to the amorphous silicon film to polycrystallize it. Polycrystallization by laser radiation can be performed at a temperature of a strain point or lower of a glass substrate and it provides the advantage that an expensive quartz substrate is not necessary.
A gate insulating film is formed to a thickness of 100 to 150 nm on the polycrystallized silicon film. On this gate insulating film, a metal film for a gate electrode is formed. By patterning the metal film and gate insulating film, the polysilicon film is exposed in the areas where source and drain regions are to be formed. Impurities are doped into the exposed polysilicon regions to form the source and drain regions.
Polycrystallization with an excimer laser beam forms protrusions along crystal grain boundaries of a polysilicon film. These protrusions are higher than that formed when an amorphous silicon film on a quartz substrate is polycrystallized by heating it in an electric furnace. Therefore, polycrystallization with an excimer laser forms relatively large protrusions on the surface of a polysilicon film.
It is difficult to form a thin gate insulating film on the polysilicon film if this film has large protrusions on the surface thereof. An electric field concentrates upon the tips of protrusions and dielectric breakdown is likely to occur. Protrusions on the surface of a polysilicon film hinder micro fine patterning.
It is an object of the present invention to provide a method of manufacturing a semiconductor device capable of forming a polysilicon film with a small roughness.
According to one aspect of the present invention, there is provided a method of manufacturing a semiconductor device comprising the steps of: forming a first layer on a surface of an underlying substrate, the first layer being made of polysilicon; exposing a surface of the first layer to an environment which etches silicon oxide, and if the surface of the first layer is covered with a silicon oxide film, removing the silicon oxide film; and supplying an energy to the first layer, the energy allowing silicon crystal to re-grow.
The root mean square (RMS) of roughness of the first layer can be made small by re-growing silicon crystals after the silicon oxide film is removed. By using this polysilicon film, a highly reliable and fine TFT can be manufactured.